This invention relates to the thermal treatment of aluminum-based articles. More particularly, the invention relates to a method for imparting improved combinations of strength and fracture toughness to an article which contains an aluminum-lithium alloy. The invention further relates to a superplastically formed, aluminum-based article having improved levels of strength.
Fuel costs are a significant economic factor in today's aerospace industry. Aircraft designers and manufacturers are constantly striving to improve fuel efficiency and overall performance. One method for effecting such improvements is to reduce the effective weight of materials used to manufacture structural components, while maintaining or increasing the strength, fracture toughness and/or corrosion resistance of such materials.
It is known to solution heat treat, quench and age aluminum alloy articles for enhancing certain physical properties. In its most natural form, aging consists of allowing the article to cool at about room temperature for a significant amount of time before further processing. It is commercially more practical to artificially age some articles for shorter times at elevated temperatures, however.
It is generally known to artificially age articles made from 7000 Series aluminum alloys (Aluminum Association designation) in two steps or stages. The first step consists of precipitation hardening the article at temperatures between about 96.degree.-135.degree. C. (205.degree.-275.degree. F.), although temperatures as high as 177.degree. C. (350.degree. F.) were suggested in U.S. Pat. No. 2,248,185. The article is then further heated at temperatures below 232.degree. C. (450.degree. F.), more preferably between about 149.degree.-193.degree. C. (300.degree.-380.degree. F.), for imparting either better corrosion cracking resistance or better strength properties to the same. Exemplary of such two-step treatment methods are those disclosed in U.S. Pat. Nos. 3,231,435, 3,881,966, 3,947,297, 4,030,947 and 4,305,763.
Multiple-step aging practices are also known for Al-Mg-Si and Al-Zn-Mg extrusions. For example, U.S. Pat. No. 4,495,001 teaches passing such extrusions through a first zone at 160.degree.-200.degree. C. (320.degree.-392.degree. F.) for 45-60 minutes, followed by treatment through a second zone at 230.degree.-260.degree. C. (446.degree.-500.degree. F.) for 10.degree.-20 minutes. U.S. Pat. No. 4,214,925 discloses a method for making brazed aluminum fin heat exchangers from Al-Mg-Si alloys. As part of this method, an alternative two-step aging practice is disclosed at FIG. 6 which includes a first heat treatment at 50.degree.-100.degree. C. (122.degree.-212.degree. F.) for at least 10 hours, followed by further treatment at 150.degree.-175.degree. C. (302.degree.-347.degree. F.) for 16 hours or more.
It is further known to thermally treat zinc- and copper-bearing aluminum alloy articles with high-to-low temperature aging processes. U.S. Pat. No. 3,305,410, for example, teaches aging such articles at a first temperature between 163.degree.-246.degree. C. (325.degree.-475.degree. F.), followed by further aging at 93.degree.-177.degree. C. (200.degree.-350.degree. F.). The foregoing method was considered especially applicable for articles made from 2017, 2024 and 7075 alloys, however. In U.S. Pat. No. 3,198,676, there is disclosed a two-step aging method which varies with the zinc content of the article to be treated. Specifically, for articles containing less than 7.5 wt. % zinc, the first step includes aging at 93.degree.-135.degree. C. (200.degree.-275.degree. F.) for 5-30 hours. For articles containing greater than 7.5 wt. % zinc (among other elements), the first step includes heating at 79.degree.-135.degree. C. (175.degree.-275.degree. F.) for 3-30 hours. Both first steps are then followed by aging at 157.degree.-193.degree. C. (315-380.degree. F.) for 2-100 hours.
In the aerospace industry, it is well recognized that the addition of lithium to aluminum often results in reduced alloy density and, thus, lower effective weight. Unfortunately, lithium additions to aluminum are not without their problems. Aside from various casting and handling difficulties, lithium additions tend to reduce an aluminum alloy's ductility and fracture toughness. Before lithium-containing aluminum alloys are used more commonly in aerospace manufacture, therefore, it is imperative to develop a method for improving both the strength and fracture toughness of such alloys.
It is known to produce a dispersion-hardenable aluminum-lithium alloy article through powder metallurgy techniques. After formation, these articles may be solution heat treated, quenched and aged at 95.degree.-260.degree. C. (203.degree.-500.degree. F.) for 1-48 hours, according to U.S. Pat. No. 4,409,038. It is further known to heat treat aluminum-lithium alloy articles by one-step aging at 93.degree.-149.degree. C. (200.degree.-300.degree. F.) as in U.S. Pat. No. 4,603,029. Further property improvements may be realized by cold working aluminum-lithium alloys to an equivalent of at least about 3% stretching, prior to aging, as taught in U.S. Pat. No. 4,648,913, the disclosure of which is incorporated herein by reference.
In Russian Pat. No. 707,373, there is disclosed a two-step method for thermally treating Al-Cu-Li-Mn-Cd alloy products. The first step consists of aging the products at 145.degree.-155.degree. C. (293.degree.-310.degree. F.) for 3-4 hours. The second step consists of further aging at 180.degree.-190.degree. C. (356.degree.-374.degree. F.) for 3-4 more hours. Russian Pat. No. 994,112 teaches a two-step method for aging extruded aluminum-magnesium-lithium components to improve the corrosion resistance thereof. The second aging step of this method requires higher operating temperatures between 400.degree.-420.degree. C. (752.degree.-788.degree. F.), however.
Lastly, it is known to exploit the spinodal decomposition characteristics of Cu-Ni-Sn alloys for improving the strength and stress relaxation resistances of such copper-based alloys. Exemplary products made from these alloys include those taught in U.S. Pat. Nos. 3,937,638, 4,052,204, 4,090,890, 4,142,918 and 4,641,976.